Liposomes can be prepared with different size, charge and fluidity. A large variety of antitumor drugs can be encapsulated. The long term objective of the project is to design liposomes as tumor-specific carriers for the antitumor drugs. In the last funding period we have succeeded in covalently coupling to liposomes monoclonal antibodies specific for cell surface antigens and demonstrated target-specific cytotoxicity of drugs encapsulated in the immunoliposome. We have also shown that immunoliposomes are endocytosed by the target cell. A new type of immunoliposome has been designed which becomes highly fusion competent when exposed to acidic pH in the range of 4 to 6. We have demonstrated that these pH-sensitive immunoliposomes can efficiently deliver their contents into the cytoplasm of the target cells. We propose to verify the mechanism of delivery is via an acid induced fusion of the liposome membrane and the membrane of the endosome, in a manner similar to the infection pathway of the Influenza and Semliki Forest Viruses. Immunoliposomes containing fusion-active peptides will be prepared for effective liposome fusion either at the endosome level (acidic pH) or at the plasma membrane level (neutral pH). Immunoliposomes which release the contents upon binding to the target cells will also be prepared. The biophysical principle of the design is based on the lateral phase separation of the antibody and the formation of the hexagonal phase by the unsaturated phosphatidylethanolamine. The role of antibody density of the immunoliposomes on the target-specific cell binding will be examined. Liposomes containing two types of antibody for different antigens on the same tumor cell will be used for increased target specificity. Killing of the tumor cells by cytotoxic drugs encapsulated in various types of immunoliposomes will be examined in vitro. In vivo cytotoxicity will be evaluated with lymphoma cells grown in the nude mice. Finally, a brain tumor model will be established in dogs for intrathecal administration of liposomes.